A corrugated box obtained by folding a corrugated sheet 50 as shown in FIG. 9 is known as a packing box for storing or moving objects. The upper lid and the base plate of a corrugated box is formed by overlapping separated parts that are obtained by using slits formed at some parts of the corrugated sheet 50.
The slits are usually formed using a grooving device. A cutting tool 60 as shown in FIG. 10 is well known for use as the grooving device (see BACKGROUND OF THE INVENTION of Patent Document 1. Japanese Unexamined Patent Publication No. 1997-39118, for example).
In the cutting tool 60, a notching blade 62 and a grooving blade 63 are integrally formed on a fan-shaped cutting tool body 61. The notching blade 62 projects outwards radially from one end of the outer surface of the cutting tool body 61 in such a manner as to be flush with the end surface of the cutting tool body 61, and is provided with an angled portion 64 on either side of the end surface in the width direction. The grooving blade 63 is formed on either side of the cutting tool body 61 in the thickness direction along the outer surface of the cutting tool body 61.
The cutting tool 60 is attached to a grooving device 70 as shown in FIGS. 11 and 12. FIG. 11 is a side view schematically showing the grooving device, and FIG. 12 is an elevation view of FIG. 11. The grooving device 70 is equipped with two cutting tools 60, which are referred to as cutting tools 60a and 60b. The construction of the grooving device is described below.
The grooving device 70 is provided with an upper rotation axis 71 and a lower rotation axis 72.
The upper rotation axis 71 and the lower rotation axis 72 are positioned in parallel facing each other across a sheet feed line L, and are provided with a pair of disk-like upper rotation holders 73 and 73, and a pair of lower rotation holders 74 and 74, respectively.
Each of the two cutting tools 60a and 60b is held between the pair of lower rotation holders 73 and 73 with a fastening member (not shown), such as a bolt. The cutting tools 60a and 60b are disposed at given intervals along the outer surface of the pair of rotation holders 73 and 73 in such a manner that the notching blades 62a and 62b face each other along the direction of the periphery.
On the other hand, the pair of lower rotation holders 74 and 74 are provided with two receiving blades 75 and 75 on their opposing sides at given intervals in such a manner as to match with the thickness of the cutting tools 60a and 60b. 
An explanation as to how to form a slit in the corrugated sheet 50 using the grooving device 70 configured as described above is given below. As shown in FIG. 11, while the pair of upper rotation holders 73 and 73 and the pair of lower rotation holders 74 and 74 are rotated, the corrugated sheet 50 is fed to the grooving device 70 from the right, as seen from FIG. 11, along the sheet feed line L of the grooving device 70. By the rotation of the holders, the cutting tool 60a is placed in the gap between the receiving blades 75 and 75 to cut the corrugated sheet 50, thereby forming a front slit 51 which extends from a slit terminal end 53 as shown in FIG. 9. In the same manner, the other cutting tool 60b is placed in the gap between the receiving blades 75 and 75 to cut the corrugated sheet 50, thereby forming a back slit 52 which extends from the slit beginning portion 54.
Patent Document 2, Japanese Examined Utility Model Publication No. 1994-1356, teaches a construction in which a cutting tool is provided with notches on either side of the cutting tool body, and notching blades are detachably attached to the notches.